Fluidized bed coating processes are used to deposit carbon (C), silicon (Si), siliconcarbide (SiC), and other ceramics and metals onto particles and components used in low observable and medical applications, for example. The fluidized bed typically is a bed of particles that are lifted and agitated by a rising stream of a process gas. At low velocities, the amount of lifting is slight and the bed behaves as a boiling liquid, hence the term fluidized bed. The bed is typically contained within a furnace, or reactor heated inductively, resistively, or otherwise.
In chemical vapor deposition (CVD) at least a part of the gas used to fluidize the bed is a reactive gas that break downs when it contacts the heated particles into coating materials such as carbon, silicon or silicon carbide. Often times, for example, methane is used, perhaps along with argon as a carrier. The term chemical vapor infiltration, or CVI, is used when the particles or articles being coated are not merely surface coated, but the reagent enters into the porous substrate and is deposited therein.
In the above described conventional fluidized bed, the entire bed and vessel are hot. Thus, the interior surface of the vessel, which is commonly made out of graphite, as well as the particles, will become coated by the reagent. As such, the vessel and the particles must be frequently replaced.
Some work has been done in the past using liquid reagents with CVD or CVI processes to achieve coating of filaments and infiltration of porous preforms. For example, U.S. Pat. No. 3,850,689 to Basche et al. teaches a process to deposit silicon carbide on filaments by passing the hot filament through a liquid organo silicon halide, wherein the temperature of the hot surface is high enough to cause the organo silicon halide immediately surrounding the hot surface to film boil, decompose, and deposit a silicon carbide coating on the filament. Two other U.S. Pat. No. 5,389,152 to Thurston et al. and U.S. Pat. No. 4,472,454 to Houdayer et al., teach processes for the densification of a porous preform that involve immersing a conductive preform in a liquid hydrocarbon or other precursor liquid. An induction coil is used to heat the preform so that the precursor liquid immediately surrounding the preform decomposes and carbon, or another element, is infiltrated into the preform. These processes are taught using a stationary preform.
Thus, there exists a need in the art for a coating method using chemical vapor deposition or chemical vapor infiltration from a fluidized bed in order to quickly, economically, and completely coat a substrate.